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Scientists have discovered asteroids can split in two if they rotate too quickly.

The revelation could help improve understanding of these floating mountains that occasionally hit the Earth.

Reporting in the journal Nature, Dr Petr Pravec of the Astronomical Institute in the Czech Republic and his team have found that sunlight falling on asteroids causes their spin to increase.

"When asteroids spin fast enough, they can undergo rotational fission, splitting in two pieces, which then begin orbiting each other as binary asteroids," Dr Pravec said.

Pravec's team found many of these binary asteroids don't remain gravitationally linked to each other. Instead, they head in different directions forming new orbits around the Sun.

Asteroid pairs

The researchers looked at 35 asteroid pairs, which are now 70 separate asteroids in orbit around the Sun. Their trajectories indicate they once came within a few kilometres of each other, some time in the past million years.

They measured the relative brightness of each asteroid pair, which correlates to its size, and determined the spin rates using a technique known as photometry.

Pravec says in each case the smaller asteroid was always less than 60 percent of the size of its companion.

The measurement fits precisely with a theory developed in 2007 by study co-author Professor Daniel Scheeres from Colorado University-Boulder.

Scheeres' theory predicts that if a binary asteroid forms by rotational fission, the two can only escape from each other if the smaller one is less than 60% of the size of the larger asteroid.

Scheeres says the initial orbit of a binary asteroid after formation is chaotic.

"The smaller guy steals rotational energy from the bigger guy, causing the bigger guy to rotate more slowly and the size of the orbit of the two bodies to expand. If the second asteroid is small enough, there is enough excess energy for the pair to escape from each other and go into their own orbits around the Sun."

It is estimated that there are over a million asteroids one kilometre across or larger, orbiting the Sun. Most are thought not to be solid chunks of rock, but rather, gravitationally bound debris piles.

Pravec says asteroids are important to understanding life on Earth.

The 10-kilometre wide Chicxulub asteroid ploughed into the Yucatan Peninsula 65 million years ago, sparking a mass extinction event that wiped out about 70% of all life on Earth, essentially resetting the evolutionary clock.

Some asteroids have even been found to contain amino acids - the building blocks of life - causing some scientists to speculate that life on Earth could have come from asteroids pelting the planet.

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